1. Field of the Invention
The present invention relates to a motor driving circuit.
2. Description of the Related Art
A sensorless method, which does not require a position detecting device such as a Hall element, is known as a driving method for a brushless DC motor which includes driving coils of a plurality of phases (e.g., three phases). In the sensorless method, a counter electromotive force which is generated in each of the driving coils is used, and a voltage of each of the driving coils of the phases is compared with a voltage of a neutral point, so that a position of a rotor (rotator) can be detected.
In addition to a PWM (Pulse Width Modulation) method, a PAM (Pulse Amplitude Modulation) method disclosed in Japanese Laid-Open Patent Publication Nos. 2000-350490, 2002-142484, and 2008-259340 is known as a control method of output transistors that supply driving currents to the driving coils, for example. According to the PAM method, a pulse-amplitude-modulated switching signal is supplied to the output transistors.
FIG. 4 depicts an example of a configuration of a common motor driving circuit configured to drive a sensorless three-phase motor by the PAM method.
In a motor driving circuit lb depicted in FIG. 4, a comparator circuit 21 compares voltages U, V, and W of the phases of driving coils 61 to 63 with a voltage COM of a neutral point. A position detecting circuit 11 detects the position of a rotor of a motor 6 based on a comparison result signal CMP. A switching control circuit 12 generates switching signals S41 to S43 and S51 to S53, which are pulse-amplitude-modulated according to a rotor position signal RP, and supplies the switching signals to output transistors 41 to 43 and 51 to 53, respectively. The output transistors supply driving currents to the driving coils 61 to 63, and the current value of the driving currents is detected as a voltage RF.
In the motor driving circuit 1b, a comparator circuit 14 compares the voltage RF with a voltage V1, and outputs a current limit signal LMT. The switching control circuit 12 limits the amplitude of each of the switching signals according to the current limit signal LMT, and limits the driving currents to a predetermined current value. Therefore, the comparator circuit 14 functions as a current limiting circuit that limits the driving currents, so that the motor driving circuit 1b can rotate the motor 6 at a target rotation speed.
In this manner, the sensorless three-phase motor can be driven in the PAM method and can be rotated at the target rotation speed.
The sensorless motor driving circuit is not able to detect the position of the rotor at the time of startup. Therefore, the switching control circuit generates predetermined switching signals so that predetermined startup currents are supplied as the driving currents. When the motor starts to be rotated by the startup currents, the position of the rotor can be detected, and therefore, the switching control circuit generates the switching signal corresponding to the position of the rotor.
However, when the target rotation speed of the motor is set at a high speed, great startup currents are supplied to the driving coils, and therefore, the motor may start to rotate at a speed faster than a speed of the switching timings of the switching signals. Therefore, the motor may lose its synchronization or may rotate in the reverse direction at the time of startup.